Modern semiconductor integrated circuits usually involve multiple layers separated by dielectric (insulating) layers, such as silicon dioxide or silica, often referred to simply as an oxide layer, although other materials are being considered for use as the dielectric. The layers are electrically interconnected by holes penetrating the intervening oxide layer which contact some underlying conductive feature. After the holes are etched, they are filled with a metal, i.e., aluminum or copper, to electrically connect the bottom layer with the top layer. The generic structure is referred to as a plug. If the underlying layer is silicon or polysilicon, the plug is a contact. If the underlying layer is a metal, the plug is a via.
Plugs have presented an increasingly difficult problem as integrated circuits are formed with an increasing density of circuit elements because the feature sizes have continued to shrink. The thickness of the oxide layer seems to be constrained to the neighborhood of 1 .mu.m, while the diameter of the plug is being reduced from the neighborhood of 0.25 .mu.m or 0.35 .mu.m to 0.18 .mu.m and below. As a result, the aspect ratios (the ratio of the depth to the minimum lateral dimension) of the plugs is being pushed to 5:1 and above.
As sizes continue to decrease, new processes are being developed to enable metal to be deposited in the small features. One process which has been developed is high density plasma physical vapor deposition referred to as an ion metal plasma process. This process is characterized in that a source coil driven by RF energy is disposed in the process chamber to ionize sputtered metal ions which are dislodged from a target in the chamber. An electrical bias connected to a susceptor on which a substrate is supported in the chamber capacitively couples energy to the substrate and redirects the ions at the substrate surface enabling small features to be filled with metal.
One problem encountered in ion metal plasma processes is that the process is preferably carried out at higher pressures than are typically used in physical vapor deposition processes. Typical ion metal plasma processes are carried out in a pressure range of about 15-30 mTorr. Higher process pressures require longer time to stabilize the gas in the chamber prior to processing and require a longer time to pumpdown the chamber prior to moving the substrate out of the chamber. Typically, a transfer chamber is connected to the process chamber via a slit opening and valve assembly and the transfer chamber is maintained at a high vacuum such as about 10.sup.-3 Torr or higher. As a result, the ion metal plasma chamber must be pumped down to a high vacuum following processing before opening the slit valve and moving the substrate into the transfer chamber.
The gas stabilization time and pumpdown time have heretofore been arbitrarily set and result in a high overhead time. As one example, FIG. 1 illustrates a titanium ion metal plasma process in which the gas stabilization time is 20 seconds, as indicated by interval 10, the pumpdown time following deposition is 10 seconds, as indicated by interval 12, and the total overhead time is 30 seconds. In this exemplary process, argon gas is flowed into the chamber at a rate of about 47 sccm during the gas stabilization step and the deposition step.
Therefore, there remains a need for a method to reduce the overhead time in a deposition process an optimize an ion metal plasma process. It would be preferable if the minimum overhead time associated with a particular chamber and process could be achieved.